This application claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 60/531,140 filed on Dec. 19, 2003.
1. Field of the Invention
The present invention relates to high SBS threshold optical fibers. More preferably, the present invention relates to high SBS threshold non-zero dispersion shifted optical fibers, or NZDS fibers, or NZ-DSF's.
2. Technical Background
Stimulated Brillouin Scattering (SBS) is a dominant nonlinear penalty in many optical transmission systems. In many systems, the launching of large power to optical fiber while maintaining high signal to noise ratio (SNR) is desirable. However, as the launch power or signal power of an incident signal launched into an optical fiber increases, the launch power may exceed a certain threshold power and part of the signal power gets reflected due to SBS as a reflected signal. An undesirably large amount of the signal power can thus be reflected back toward the transmitter due to SBS. In addition, the scattering process increases the noise level at the signal wavelength. The combination of decrease in signal power and increase in the noise both lower the SNR and lead to performance degradation.
At finite temperatures, thermal excitations occur in glasses similar to phonons in crystals, and the interaction of these vibrational modes with low intensity signal light produces spontaneous Brillouin scattering. An intense optical field generates pressure or sound waves through electrostriction due to the beating of intense incident and spontaneous reflected light giving rise to pressure or acoustic waves. The change in pressure causes material density to change, thereby resulting in refractive index fluctuations. The net result is that an intense electrical field component of the optical wave generates pressure or sound waves which cause density fluctuations. The acoustic wave changes the refractive index and enhances the reflected light amplitude through Bragg diffraction. Above the SBS threshold of an optical fiber, the number of stimulated photons is very high, resulting in a strong reflected field which limits the optical power that is transmitted and which reduces the SNR.